1. Technical Field
This invention relates to the technology of surface preparation of metal surfaces for wear resistant and high temperature thermal spray coatings, and more particularly to electrical discharge sparking for preparing such surfaces.
2. Discussion of the Prior Art
The prior art has essentially used three types of techniques for preparing surfaces for thermally applied coatings, namely, grit blasting, water jetting, and high frequency magnetic field agitation. Surface preparation, prior to coating deposition, is necessary to ensure adequate coating adhesion; this occurs principally through a mechanical interlocking when the molten spray coating impacts the surface, solidifies and conforms to the surface topography of the component being coated.
Grit blasting requires that the fluidized supply of grit particles be impacted against the target surface with fluid pressures which are relatively high, greater than 50 psi. The grit mediums for such surface preparation usually consist of silica sand, aluminum oxide, chilled iron or garnet particles to produce the roughened surface. Unfortunately, with grit blasting the surface must be cleaned of any foreign surface contaminants, residual grit and dust before the thermal spray coating is applied. Moreover, grit blasting is not suitable for high-volume on-line manufacturing of thermal spray coated components for several reasons: (i) grit contamination of components and process equipment is a problem in high volume manufacturing environments, (ii) on-line collection and containment of the grit medium is difficult, it is not an environmentally clean process, (iii) the techniques for grit blasted surfaces do not allow for accurate dimensional control, (iv) grit blasting is a line of sight process making surface preparation of complex components extremely difficult, with poor reproducibility.
Water jetting has been utilized to clean cylinder wall surfaces of an aluminum engine block by a blast of high pressure water. The water is directed against the cylinder wall surface by a rotating water spray nozzle axis which is lowered and raised along the axis of each cylinder. The disadvantage of water jetting is that it abrades small shallow pits in non-ferrous metals, such as cast aluminum alloy, roughening only to a degree of about 50 micro meters. The cost for water jetting is extremely high and requires expensive high pressure water pumps.
High frequency electric current agitation has been applied to metal, particularly sheets, at extremely high levels between 50,000 and 600,000 volts at a frequency between 25-400 Hz to clean and activate the metal surface for improving the adherence of inks, paints and plastics. The process is more akin to ultrasonic cleaning in that it disrupts the tenacious native oxide layer on the surface of metals to allow a better coating adhesion, but does not erode or affect the metal surface itself.
Electrical discharge devices have been used heretofore to machine or cut metal surfaces to produce as smooth or satin-like surface as can be obtained. High voltages at low amperages for considerably long "on" periods have been used to produce a cut surface changed little in hardness. The technique simply removes metal from the work surface in a smooth, satin-like manner.